1. Field of the Invention
This invention relates generally to geophysical exploration. More particularly, the invention relates to a novel method for generating a seismic signal.
2. Description of the Prior Art
Reflection seismology is a well-known technique for prospecting for subsurface oil and gas reservoirs, both on land and in marine environments. A seismic source is utilized to generate acoustic waves, normally at or near the earth""s surface, and these waves travel downwardly into the earth""s subsurface. A portion of the seismic wave energy is reflected or refracted from the subsurface interfaces between earth strata having different acoustic velocities, and this reflected or refracted energy is then detected by sensors which are normally deployed at the earth""s surface. The detected signals are normally recorded for later signal processing. The travel time of the seismic signal from the seismic source location down to various subsurface interfaces is determined and this travel time along with a velocity profile of the earth""s subsurface are utilized to determine the subsurface location of these reflecting interfaces. The velocity profile may be determined from the seismic data or it may have been predetermined. Subsurface acoustically reflecting interfaces often correspond to the location of an oil and gas reservoir.
A wide variety of sources have been used, including dynamite, mechanical impact sources and seismic vibrator sources. A wide variety of impact sources are known, including by way of example, but not limited to, those described in U.S. Pat. Nos. 4,124,090; 4,421,198 and 4,011,924.
The prior art shows certain uses of aircraft in geophysical exploration. It is known, for example, to conduct electromagnetic and gravity surveys with overflying aircraft carrying electromagnetic and gravity sensing instruments. It has also been proposed, in U.S. Pat. No. 4,223,759, to transport seismic sensors to a survey site by means of an aircraft and to drop the sensors to the earth""s surface as the aircraft overflies the survey site.
There has been a long felt need for a system for conducting seismic exploration that reduces the impact on environmentally sensitive areas such as the arctic region and land-marine transition zones. There are also regions, such as the marine-land transition zones and mountainous regions, that are difficult to traverse with a transport vehicle. Accordingly, it is an object of this invention to generate a seismic signal without requiring surface transportation of the means for creating the seismic signal to the location where the signal is to be generated.
In an embodiment of the invention, a seismic signal is generated at a survey site by dropping a mass from an overflying aircraft so that the mass will impact the earth""s surface at the survey site. Seismic sensors are deployed within the survey site to detect seismic signals resulting from the impact of the mass on the earth""s surface.